The purpose of the NIMH Psychoactive Drug Screening Program is to provide screening of novel synthetic compounds and natural products for potential use as research tools or probes for basic and clinical research, as therapeutic agents for mental disorders, and as PET, SPECT, and fMRI ligands for functional brain imaging. This program is primarily intended as a screen for compounds that have previously been shown to possess pharmacological, biochemical, or behavioral activities relevant to NIMH and not for the purpose of large-scale, random screening of natural products or combinatorial libraries. However, limited screening may also be conducted on small libraries of synthetic or natural product compounds of unknown activity and on hit or lead compounds generated through NIH-funded HTS or chemistry programs. Compounds will be provided by the research community, primarily NIH-supported investigators. It is anticipated that the Contractor will receive and screen compounds on a continuous and on-going basis, although the volume of requests will vary from month to month.